1. Field of the Invention
The present invention relates generally to expandable downhole tools. More particularly, the present invention relates to a concentric expandable downhole tool having fewer components and thus a shorter length than conventional expandable tools. Still more particularly, the present invention relates to a robust, concentric expandable reamer having an advanced cutting structure and a mechanical/hydraulic activation mechanism.
2. Description of the Related Art
In the drilling of oil and gas wells, a plurality of casing strings are installed concentrically and then cemented into the borehole as drilling progresses to increasing depths. Thus, each new casing string is supported within the previously installed casing string, such that the largest diameter casing string is disposed at the uppermost end of the borehole and the smallest diameter casing string is disposed at the lowermost end of the borehole.
As successively smaller diameter casing strings are suspended, the annular area between the casing and the borehole wall is increasingly limited for the cementing operation. Further, as successively smaller diameter casing strings are suspended, the flow area for the production of oil and gas is reduced. Therefore, to increase the annular space for the cementing operation, and to increase the production flow area, it is often desirable to enlarge the borehole below the terminal end of the previously cased borehole. By enlarging the borehole, a larger annular area is provided for subsequently installing and cementing a larger casing string than would have been possible otherwise. Further, by enlarging the borehole, the bottom of the formation can be reached with comparatively larger diameter casing, thereby providing a larger flow area for the production of oil and gas.
Various methods have been devised for passing a drilling assembly through an existing cased borehole and enlarging the borehole below the casing. One such method includes using a winged reamer behind a conventional drill bit. In such an assembly, a conventional pilot drill bit is disposed at the lowermost end of the drilling assembly with a winged reamer disposed at some distance behind the drill bit. The winged reamer generally comprises a tubular body with one or more longitudinally extending “wings” or blades projecting radially outwardly from the tubular body. Once the winged reamer has passed through any cased portions of the wellbore, the pilot bit rotates about the centerline of the drilling axis to drill a lower borehole on center in the desired trajectory of the well path, while the eccentric winged reamer follows the pilot bit and engages the formation to enlarge the pilot borehole to the desired diameter.
Another method for enlarging a borehole below a previously cased borehole section includes using a bi-center bit, which is a one-piece drilling structure that provides a combination reamer and pilot bit. The pilot bit is disposed on the lowermost end of the drilling assembly, and the eccentric reamer bit is disposed slightly above the pilot bit. Once the bi-center bit has passed through any cased portions of the wellbore, the pilot bit rotates about the centerline of the drilling axis and drills a pilot borehole on center in the desired trajectory of the well path, while the eccentric reamer bit follows the pilot bit and engages the formation to enlarge the pilot borehole to the desired diameter. The diameter of the pilot bit is made as large as possible for stability while still being capable of passing through the cased borehole. Examples of bi-center bits may be found in U.S. Pat. Nos. 6,039,131 and 6,269,893.
As described above, winged reamers and bi-center bits each include reamer portions that are eccentric. A number of disadvantages are associated with this design. In particular, due to directional tendency problems, these eccentric reamer portions have difficulty reliably enlarging the borehole to the desired diameter. With respect to a bi-center bit, the eccentric reaming section tends to cause the pilot bit to wobble and undesirably deviate off center, and any off-center rotation will cause the reaming section to drill an enlarged borehole that is undersized. A similar problem is experienced with respect to winged reamers, which only enlarge the borehole to the desired diameter if the pilot bit remains centralized in the borehole during drilling. Accordingly, it is desirable to provide a reamer that remains concentrically disposed in the borehole while enlarging the previously drilled borehole to the desired diameter.
There are several different types of concentric reamers, which are used in conjunction with a conventional pilot drill bit positioned below or downstream of the reamer. The pilot bit drills the borehole while the reamer follows to enlarge the borehole formed by the bit. One type of concentric reamer is a fixed-blade reamer, which includes a plurality of concentric blades (sometimes also referred to as arms) with cutters on the ends extending radially outwardly and spaced azimuthally around the circumference of the reamer housing. The outer edges of the blades contact the wall of the existing cased borehole, thereby defining the maximum reamer diameter that will pass through the casing, and also defining the maximum diameter of the enlarged borehole. Thus, although a fixed-blade reamer remains concentrically disposed as it rotates to enlarge the borehole, it is limited to enlarging the borehole only to the drift diameter of the existing cased borehole, whereas winged reamers and bi-center bits can enlarge the borehole beyond the drift diameter of the casing. Accordingly, a fixed-blade reamer often will not enlarge the borehole to the desired diameter.
More recently, concentric expandable reamers have been developed. Most expandable reamers have two operative states—a closed or retracted state, where the diameter of the tool is sufficiently small to allow the tool to pass through the existing cased borehole, and an open or expanded state, where one or more arms with cutters on the ends thereof extend from the body of the tool. In this latter position, the reamer enlarges the borehole diameter to the required size as the reamer is rotated and lowered in the borehole.
Expandable reamers are available in a variety of configurations, each having different activation mechanisms and blade configurations. One type of expandable reamer includes hinged arms with roller cone cutters attached thereto. This type of reamer may utilize swing out cutter arms that are pivoted at an end opposite the cutting end of the arms. The cutter arms are actuated by mechanical or hydraulic forces acting on the arms to extend or retract them. Typical examples of this type of reamer are found in U.S. Pat. Nos. 3,224,507; 3,425,500 and 4,055,226, and they have several disadvantages. First, the pivoted arms may break during the drilling operation, requiring that the arms be removed or “fished” out of the borehole before the drilling operation can continue. Accordingly, due to the limited strength of the pivoted arms, this type of reamer may be incapable of underreaming harder rock formations, or may have unacceptably slow rates of penetration. Further, if the pivoted arms do not fully retract, the drill string may easily hang up when attempting to remove it from the borehole. Therefore, it would be advantageous to provide a reamer that is more robust and has improved blade retraction mechanisms.
Other expandable reamers are activated by weight-on-bit to extend the blades. With such designs, the internal components of the reamer rather than the reamer body support the weight of drilling assembly components extending below the reamer. Accordingly, if too much weight is applied to the internal components, the reamer may not have enough hydraulic power to lift the weight below the reamer, and the reamer will not open. Further, it may not be possible to set weight-on-bit when the reamer should be activated to extend the blades. Also, during drilling, the weight-on-bit is sometimes unevenly distributed, and a false indication may be provided to the surface that the reamer blades are expanded when they are not.
Still other types of expandable reamers are activated by hydraulic or differential pressure, sometimes in combination with a mechanical component. With such designs, there is no certainty that all of the blades will be fully extended because the blades do not activate in unison. Therefore, one blade might extend while another blade is stuck in a partially extended position. Further, in some embodiments, drilling fluid pressure is the only force holding the blades in an extended position. Thus, if the strength of the formation is greater than the fluid pressure, the blades will partially retract and drill an undersized borehole. Some embodiments include a mechanical component, such as, for example, a piston with a continuously tapered surface that engages the blades to drive them radially outwardly as the piston moves downwardly. In such embodiments, the piston is activated by hydraulic pressure to drive the blades radially outwardly, but if the strength of the formation is greater than the fluid pressure, the blades will tend to retract along the continuously tapered surface. Thus, existing expandable reamers raise such concerns as whether the tool will expand to the desired borehole diameter when required, whether the tool will remain in the expanded position to enlarge the borehole to the desired diameter, and whether the tool will reliably retract prior to re-entering the casing as the drilling assembly is removed from the borehole.
Further, most expandable tools include a large number of moving parts, thereby increasing the probability of malfunction. The number of moving parts also affects the tool length, which may be up to 14 feet long, for example. There are also disadvantages associated with existing reamer blades. Specifically, to adjust the expanded diameter of the reamer, the entire arm must be removed and replaced, or in some cases, a different reamer may be required. Further, most blades fail to include pads on the gage configuration for stability and durability, or if pads are included, the blades fail to include active cutting structures near the pads.
The present invention addresses the deficiencies of the prior art.